what is that little plug that plugs from either the intake manifold or the tbi unit into the little cap looking thing on the neck of the air cleaner on my 90 RS?

Thermac vacuum hose?

picture please, or find one and point it out

Since you never replied Ill go ahead and stab it, Im assuming thats the vacume hose for the air warmer, lil cap looking thing on the air cleaners neck, with a steel tube running to the exhaust. When the cars cold it puls a vacuum and opens this so the air coming inside the intake is warmer, then it closes it once it warms up.

Ahh alright, thanks alot, you got it spot on, sorry for the lack of pics, but now to my next question, does this affect the performance of the intake at all?

Only warm up time. It closes once the vacuum drops from the engine getting lower. Mine only gets hooked up for emissions cuz im running Open Element.