ok, so on the stock radio for a 1996 blazer s10, there are 4 spots for a harness to plug into. i only have 3 plugged into it. is the fourth an extra, like the radio is used for a car that uses all four? or am i supposed to have a fourth plugged into it?

 

I think, correct me if I'm wrong someone, but the 4th plug is used on the units with amplifiers like in Bravadas and such, which the S10's will not have, so no need for that 4th plug

my father's 95 Sonoma only had 3 of the 4 when I hooked up a radio in it for him

 

The 4th plug is for an optional add-on like an off-board CD or cassette player that mounts in the lower console. That's the same in all GM vehicles in the similar year range.

 

Ah okay that makes since