Your '91 has the newer 2 wire thermistor circuit [ 105-107 C cut-in temp. Approx. 265 ohm]
An added note.
If the low speed fan works by jumpering the a/c pressure switch with a/c on, then it is advisable to check freon level, as a low level will not allow the high side pressure to reach the cut-in pressuer required to trip the switch.

[ in other words, don't codemn the switch until you know the level]