I have been struggling to get my 1991 S10 blazer lights to function properly. --- 1-first none of the turn signals worked so i replaced the multi function switch, as a result the left blinker blinks fast and the right stays on solid. Hazards on, both will blink as normal. Somewhere along the process also i lost all my tail lights. With lights on the headlights work but not tail lights.

what would be your first step/what might be the issue?

 

I would start by checking all the grounds, if they are all good, start checking all the sockets on each function with a DMM (digital multi meter) and make sure the voltage is consistent between them all.
Grounds on these things can really cause some weird issues.
The fact that they work with the hazards on is odd, and the fact that after you replaced the multi function switch some started working and others did not, makes me think ground.

The easiest grounds to get to, are the ones for the fronts on the fire wall, one is hidden behind the battery, not a huge deal to get the battery out, the other is on the drivers side behind the air filter. You will have to remove the horn for that one, also not a huge deal.

as for the grounds in the rear... I will have to check my service manual for that one and get back to you if you do not find it before I do.