Electrical problem - '90 Chev K1500

Wondering if anyone can help...

Got a problem with the tail lights on a 1990 Chevy K1500 that started about 1.5 weeks ago. Brake lights and turn signals are also affected. Here are the symptoms:

Headlight switch on: tail lights/brake lights, turn signals (front or rear) not working. Turn signals blink one time and then quit.

Headlight switch off: brake lights and turn signals work if not used at same time.

Using turn signals with brake pedal depressed causes alternating blinking of tail lights.

[EDIT] Found out one more symptom tonight...when the tranny is in reverse, the tail lights come on. Back-up lights stay off.

I replaced the headlight switch today thinking that may be contributing to the problem. Also did this because about a month ago, I noticed a small wisp of smoke coming from the headlight switch. Only saw it once, and no problems occurred until the current issue.

One other thing to mention: I have wiring for a trailer light connector spliced in to the rear light wiring, and had to re-do this about 2 months ago. Used standard splicing connectors.

Would like to try and fix this myself. Thanks for any advice.

WC