Also double check the bulb part numbers, cars this old can get a mix (one tail light pulled and bulbs replaced, but not the opposite tail). In this instance, I've seen 1157's installed when the correct bulb is 2057.
This mix-up can cause issues like you are having. The bulb fits, but the filaments are different. I learned this the hard way when I replaced a front turn signal bulb on one side only with a brand new 1157 I happened to already have on hand and I thought "why spend more money?". End result? The brand new bulb would NOT work.
Always reference the fiero sails bulb page to verify the correct bulbs bought and installed.Fiero sails bulb guide