1998 Chevrolet Lumina Parasitic Drain

I'm not sure where or how you're checking, but yes, 0.156 amps is 156 milliamps, and that's too much. Chrysler says that with 35 ma, the battery will start the engine after sitting for three weeks. Unless specified differently by the manufacturer, that is pretty much the industry standard. Cadillac is one notable exception due to the insane need to hang a computer onto every conceivable part of the car that never needed a computer before. They specify 50 ma is okay.

There's a couple of problems though with measuring current drain on cars with computers. First of all, some of them have to time out and go to "sleep mode" after as much as 20 minutes. Until then they can draw over three amps. You have to do any current testing after that has occurred. Second, all volt / ohm / amp meters use "break-before-make" range switches and you may have to move the positive test lead to a different jack for some of the current ranges, (particularly the 10 amp scale). Switching ranges momentarily opens the circuit, and a computer will respond to that by waking up again and drawing that high current. You start out on a high current range to avoid blowing its internal fuse, then, to get more accuracy, you must switch to a lower range. You need to use a jumper wire to short out the meter while you change the range. That will prevent a computer from waking up.

Once you narrow down the excessive current by removing a fuse, you'll want to reconnect the jumper across the meter, switch back to a higher range, THEN reinstall that fuse, remove the jumper, and observe the reading. Doing it in that order will avoid blowing the meter's 2 amp fuse if you woke up a computer on that circuit. If the reading is still low, connect the jumper again, then switch back to the lower scale. What this boils down to is you can remove a fuse to verify that's the circuit that makes the current draw go away, but it's hard to double-check by putting the fuse back in. That's where many people become confused.

Once you identify the fuse for the offending circuit, narrow it down by looking for smaller fuses it feeds and test each of those circuits. If this is the last fuse, you'll only have a few things to look at. One of the more common causes on GM products is the radio display failing to turn off. The clock will turn on for a few seconds when you press the volume knob, but if it stays on it will kill the battery in a few days. Trunk and under-hood bulbs draw more than half an amp, (500 ma), so bulbs are not likely suspects in this case.

Sorry to leave you sitting for so long. Just got home.

I don't have a wiring diagram to look at, but usually there's two or more 12 volt feed wires to the ignition switch, and each one feeds multiple switches within the assembly. Instead of the switch itself, it's more likely something on that circuit is drawing the current. To verify it, unplug the switch to see if the draw goes down. If it does, check for overheated and blackened terminals in the connector first. If any arcing or corrosion occurred it will leave a carbon trail behind which will conduct some current.

Dandy. Happy to hear you found it, but you need to go further. Those blackened terminals must be replaced. They have lost their tension, and along with the arced areas, that will create resistance which translates to heat when current flows through the connection. That heat will migrate into the switch and degrade the new contacts. Normally there's only two terminals that get overheated. They can be cut out of the melted plastic body, then new ones can be plugged in individually. You can use crimp-on terminals but solder them too for a better connection.

You'll also see about 4" of those wires are hardened from being overheated. They'll be stiff and hard to bend. Solder won't adhere to them. Cut that 4" off, then splice on a new piece the same gauge. I like to push the strands into each other, then I solder them and seal the connection with heat-shrink tubing. Never use electrical tape. It will unravel into a gooey mess on a hot day. Be sure there's no sharp points of wire sticking up before you heat the shrink tubing. An end of a wire can poke through and cause a short.