I just fixed my 88 MJ......maybe yours is the same....take out the radio, remove the top and bottome covers (I'm not sure which one is the correct one). Look right at the top edge of the display that is exposed by the cover removal. On mine, there was a tiny elongated bulb. You can see where the two end contacts for the bulb are soldered. Check across the two bulb leads for power, if you have power, but no light, the bulb is blown....
Go to radio shack and ask them for their smallest 12V lamp..they are about the size of a large grain of rice and cost .89 for two in a pack.
I twisted the leads of both of these bulbs together to put them in parallel, and soldered them in....worked like a champ.