Not sure about 92 but on my 03 all instrument cluster bulbs and heater control bulbs are on the same circuit so its most likely those 2 or 3 bulbs but id go ahead and replace them all. To get to it you have to remove the trim bezel around the cluster (be careful because they like to hide screw under surrounding pieces). On my 03 i had to remove radio bezel and kick panel to get to the cluster and then for me the cluster had 4 bolts holding it in. Should be similar for your 92. Also i think you have cable driven speedo so i would avoid disconnecting it completely just kinda wiggle it around to access the bulbs they should be twist lock the white caps i believe are illumination bulbs the others are for blinkers and such. Mine were standard 194 bulbs but your local parts store should be able to look them up.
New DD-2009 Ford Mustang GT/CS candy apple red.
(sold)2003 XLT Tan 3.0L V6 5-speed, Lots of mods.