1999 GMC Truck radio juction block cut off

What was in there for a radio before? I don't know what you're referring to by "junction block".

Are the original radio plugs still in there or did someone cut them out previously?

GM has a whole pile of very customer-unfriendly business practices that force you to go back to the dealer, but that didn't really get to be a problem until 2002 on a few truck models. Their daytime running lamp circuits are another nightmare. There's about a dozen light brown wires that all look the same. Pity the people at the body shops trying to piece together a crashed car with a bunch of crushed and cut wires that all look the same.

Something that might help is to use a test light first to find the 12 volt memory wire that has voltage all the time, then the "ignition 12 volt" wire from the ignition switch. It will only have 12 volts with the ignition switch turned on or to the accessory position.

If there was an original amp, that had to be bypassed for the radio you removed, so if there was a "switched 12 volt" wire for an amp / power antenna circuit, that was likely cut and taped up already.

Original GM radios have "speed-sensitive volume" where the volume goes up when you increase speed. To run that there's two data buss wires that connect to every computer on the truck. Those will likely be a different color but they're likely also cut and taped up. No aftermarket radio uses them.

Of the remaining wires, there should be eight for the four speakers. If the original plugs are still in the dash and were used with an adapter harness, those eight wires are in two plugs, each with four wires. One is a light green and its mate is a dark green. There is also a light blue and dark blue for another speaker. I can't remember the other four colors but I have a swap meet handout with the colors listed that I can post if I can find it.

If the plugs were cut off, you can use a 1 1/2 volt "D", "C", "AA", or "AAA" cell to find which ones are for which speaker. Grab two wires and hold one on each end of the cell. When you find a pair, you'll hear a little scratching in that speaker. That way you'll know which speaker those wires are for.

You missed my point about the power antenna. That is the name of the wire coming out of the original radio. It can also be called "remote amp", but regardless of its name, it doesn't mean you have to have those options on the vehicle.

I forgot to mention that there's going to be two radio wires related to the dash lights. One is tied to the tail lights and tells the original radio's display to dim when the tail lights / head lights are turned on. The second wire is tied to the dash lights and tells the display how much to dim. Most commonly what happens is someone cuts those wires and they touch ground or don't seal the ends of those wires and they vibrate later and touch the grounded radio case, and blow a fuse. There's a fuse for the dash lights that I think is inside the truck, but that is fed off the tail light fuse, and that one might be under the hood.

You will have a red, 10 amp "Gauges" fuse inside on the left side of the dash. I can't remember for sure, but I don't think that affects the lights. If you can run the engine and the generator is working, the "Gauges" fuse is okay.

Be sure the instrument cluster is fully seated. If the mounting screws are removed, the cluster will move toward you a little. That will allow the connector to come apart. It's just a flexible printed circuit on the dash that the cluster plugs into, and it doesn't take much to have bad connections.

You don't have to connect wires behind the radio to make the tail lights or dash lights work. You have to prevent them from touching anything grounded.

The only wire colors I have are for the original GM radio. This is a drawing I made to hand out at an old car show swap meet.

I had a '98 GMC truck at my community college that I think is the same as yours. The GM dealers couldn't fix it so they donated it to us. I put some electrical "bugs" in it and some had to do with the "gauges" fuse. I'm on a borrowed computer right now, and thanks to a major house fire, I'm sitting in the library parking lot using their wireless internet. I have the dash gauges information on an older computer at home. I'll try to find that tomorrow so I can tell you what to look for.

All I can remember is it was a red, ten amp fuse, and along with the gauges, it fed the "Battery" lamp circuit that got the generator started. The students ran into additional problems when the instrument cluster wasn't bolted in tightly. The connector is rather unusual and the cluster has to be held in for it to stay connected.

Most likely I can't help with your new radio but I can offer some suggestions. Tell me the brand and about how old it is. I only repair original equipment radios and I specialize in Chrysler stuff, but I do have some installation manuals for a few aftermarket radios.

As a general rule, the 12 volt memory wire is pink. It needs 12 volts all the time to maintain the station presets and clock. The ignition 12 volts from the ignition switch is red. Those two turn the radio on. Once the radio is on, the switched 12 volts wire will have 12 volts on it to power an amp or power antenna if they're used. That is usually dark blue but it could be yellow. If you have any double wires, those will be for the speakers. Most commonly I see a pair of gray wires, white wires, dark green wires, and light green wires. You could also have two dark green wires for a front speaker, and two dark green wires with stripes, (tracers), for a rear speaker.

Some less-expensive radios only have six speaker wires instead of eight. When there's eight, two are for each speaker, one in each corner. When there's six wires, one goes to each of the four speakers, then one splits and goes to each right speakers' second terminal, and the other one splits and goes to each left speakers' second terminal.

In all original radios, and all but the cheapest aftermarket radios, no speaker wire can go to ground like they did in most cars in the '70s. Touching any speaker wire to a metal part of the vehicle will usually damage the radio.

Those green wires have to be for something else. The original radio only had one dark green wire for one of the speakers.

In the replacement radio, the constant 12 feed for the memory will be the pink or yellow wire. In this case I think it's the yellow one. They put a fuse in there because in some vehicles it will get connected to a tap in the fuse box that isn't protected by its own fuse.