Yea, i noticed when i tore out my Pimp-daddy Alpine system, circa 1991, that i had 2 antennae leads.
On my RX7, i had 2 also. I disabled the one in the rear and removed it and ran my clarion off the windwheild antenna. Its obvious, you can see the antenna in the glass, its a small defroster line, almost like a hair. My reception w/ Only the glass antenna wasnt noticably bad but then again, Clarion has the SUPER TUNER!
btw, is the 91 supposed to have an stock amplifier under the pass seat??? cuz my 6-speaker s ystem doesnt have one and im worried that i wont be able to get an aftermarket stereo to work.