With out sounding as if I am insulting your intelligence, yet just talking about some similar things that have happened with my vehicle that is the same as yours, but I've found most often they are not nessecarily related.Vehicle electrical problems never have absolute answers, other then maybe for someone who is working on them so much that a pattern developes for them, so they can recognize them quicker.I have experienced both I/P and taillight problems that are not related at all, but yet they are somewhat related through the ground system, and the (BCM) Body control module which is also associated with the day time running lights, which is realted to both systems. If it is anything to do with that module, all is not lost, because there are external connections, relays, and fuses to patiently check out before condemming the module.I've regularily found my I/P lights not working at all, because I have accidentaly changed the position of the I/P dimmer switch while I have turned the courtesy lights on and off, and sometimes I may have just brushed the switch wheel while reaching for the side window heater vent.The override switch in the switch assembly up by the rear veiw mirror, also might do things that you might not think of as well.I've found and corrected GM vehicle's electrical problems for my self and others, since I owned a 1968 GMC half ton thirty five years ago, and many of them have been nothing but grounding problems. like the half ton.It took me four hours of working outside in the Canadian winter to find the trouble with that first vehicle, but now I go straight to checking the ground when ever the tail lights are not working.Most of the times it has paid off, with considerable saving of time and frustration.On that vehicle, I literally cut off each connector and soldered the wires together on every plug connector starting at the front of the vehicle, and I worked back to the tail lights.When I got there I still had no tail lights, but Because I brought my ground with me as I worked to the back of the vehicle my ground problem jumped right at me as soon as I touched that ground to one of the tail light grounds. As I think back to that I think I just touched the metal on one of the tail light assemblies, and that light came on. With that one I took drastic measures and ran a ten gauge wire right straight from the negetive cable at the battery to each tail light.I sold that truck to my brother twenty five years ago, and he still happily drives it on holidays with his family, and every day to work. and he has never changed that, and he has never had any problem with the lights.The worst of it is, often you can't see the bad connection, they seem to get a skin on them that prevents proper connection. A little emery cloth sure brightens up an electrical connection.My most recent was with the tail light and signal lights on a vehicle the same as yours with similar problems, and it was a ground problem on the passenger side tail light of the vehicle.My ground problem not only caused loss of proper taillight and signal light operation, but it was also causing the taillight on the driver side to have erratic operations in response to the signal light operations meant for the passenger side. Note not even the signal light on the driver side, but the tail light bulbs.It was only because I've had several instances of ground problems that I start my troubleshooting with that in mind now.++++++++++++++++++++++++++++++Remove both tail lights from the vehicle, but leave the wires hooked up.Turn on all the switches that are supposed to make the tail lights work and I often turn on a signal light, and sometimes I will even turn on the hazzard lights to get a preliminary analysis.Go to one of the tail lights. and since none of them are presentally working, it doesn't really matter which one you start with.The first idea is to find out if there is actually power coming to the lights, and in the same instance you will be finding out if the ground is the problem.I use a test light with long wire clipped on to the frame underneath the vehicle, and I back probe the bulb sockets. Don't rely on a ground in the fender until you have checked it out.Since the test light has a bulb, it doesn't matter if you accidentally touch a live wire of one of the bulbs. In fact it is a good idea to deliberately do that anyway.If it is a ground problem, the test light will light, because now you are providing a good ground for it and a power source for it from the tail light bulb socket.The tail light bulb may even dimmly light up as well.If that happens, it is almost sure that you have a ground problem. When I found mine, I simply cleaned all the connections where ever I could get at them.That was about eight thousand kilometers ago and and everthing is still working correctly.