I think the correct bulb is a 1156 or 1157 (i think 1156 is correct and you can usually get them in amber colors). You shouldn't be having any problems getting them in any parts store. I think the actual number that's standard for an amber bulb that would fit is #1156a. It shouldn't be hard to find a bulb. Another thing you could do is to find a junkyard mustang from '87+ and cut the socket out and use the bulbs from the later model (3156). good luck. Hope this helps!
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'84 Mustang 5.0 LX
My car