Where is the tail light relay?

There is no such thing as a '91 Dodge 600 and no such thing as a tail light relay except in some newer truck, for trailer towing packages.

Those are two totally different vehicles but they do share some circuit designs. The Colt is one of the few Chrysler products I don't have a service manual for. It's actually an import car that Chrysler just put their name on.

Can I assume you have no tail lights? If so, what about dash lights and the front running lights? If all of those are dead, start by checking for a blown fuse.

Test right at the head light switch. One wire will have 12 volts all the time. That one has to be good if the head lights work. A second wire will have 12 volts only in the head light position. Ignore those two. A different one must have 12 volts in the tail light position. That wire also feeds the rheostat for the dash lights. Since the tail lights and the dash lights are out, we know it's something in common with both of them, and that would be that wire. Check for overheated, (blackened) terminals and a part of the connector melted.

Actually that's something I can't help with even though I'm an electrical expert. I worked at a very nice family-owned Chrysler dealership through the '90s but we didn't see many Colts, and I never disassembled a headlight switch on one. Your best bet is get a copy of the manufacturer's service manual. That will list all the steps along with drawings. Check first on eBay. I bought a lot of electrical manuals from a seller called "lorieandjeff" for my Automotive program. I think they go by "blackhatauctions" now.

There are online service manual companies too. You'll find a link here to Mitchell On Demand. All Data is another one.

Rather than trying to dig down to the switch terminals, look for where the wiring harness runs up the steering column. There is almost always a connector that allowed the column to be installed and plugged in on the assembly line.

Back-probe those wires and look for one or two that have 12 volts all the time. Of the remaining wires, at least one mush have 12 volts when just the running lights are turned on, and at least one different one must have 12 volts when the headlights are turned on. If no wire gets 12 volts in one of those positions, you can be pretty sure it's a switch problem. If you DO get 12 volts in every position, that may not be conclusive because there may supposed to be two wires that get 12 volts in one position. In that case we don't know for sure without a diagram to look at. A possible example would be one tail light wire gets 12 volts in that position for the tail and running lights but a second wire goes to the dash light dimming rheostat.

Don't forget too that when the high / low beam dimmer switch is built into the headlight switch, there's going to be two different wires that get 12 volts, one when the low beams are switched on, and one when the high beams are switched on.

Another plan of attack is to remove a headlight switch from a car in one of the pick-your-own-parts salvage yards. If you're anywhere between Ohio and southern Georgia, do a search for "Pull-A-Part". I've been to 16 of their 23 yards. All are very clean and well-organized. Employees and customers have always been real friendly and helpful, and parts are very inexpensive. You pay a dollar admission, throw your toolbox in one of their wheel barrows, and you can spend all day there. That will give you the opportunity to use trial and error to see how the switch comes out without breaking things on your car.