try checking resistance of ignition switch, you could have a bad spot in the switch somewhere.
The green color antifreeze will be just fine.
Probably in need of a complete tune up.
my 1997 Chevy k1500 well run fine and then it trips fuel pump relay
Mercon works fine.
automatic trans fluid. Mercon works fine.
it means everything is fine, code 12 denotes that system is operating fine...
1988 through 1995 will work just fine.
Do not install the shims. Your starter should work fine.
bad light switch.
It would depend on your environment. If it stays well above freezing, straight 30 weight is fine.
If you are not towing you should be fine.
the heatercore is most likely plugged up; get a new one; they are about 50 bucks; the ducts could have fallen off or rotted away but it think your heater core is plugged; a flush MIGHT help but a flush after your new heater core is installed would be money better spent