digital speedometer does not work. why on 1990 Mercury Cougar

do not know diagnostic code, how often, just bought it fromna private party and the digital speedmoeter does not work
need to get it fixed but do not know where to start.

Asked by for the 1990 Mercury Cougar
Does it light up at all, or is it blank? If it is lit up and reads "0" you could have an issue with a vehicle speed sensor or wiring. If it is not lit up, you could have illumination bulbs burned out, a blown fuse or an issue with the instrument cluster itself. Hopefully it is not the cluster itself, as finding a replacement at this point will be difficult.
it helps me, I'm working on a 87 cougar with about the same problem, I guess I will pull out the cluster, clean all the contacts, and go from there, it doesn't light up at all.
Yeah - anonymous - it doesn't look good for your '87. You'll probably need to do some digging to find a good used one out there somewhere...