My '89 Crown Vic keeps blowing brake light fuses. Does fine until I use cruise control then blows fuse. Still has parking lights and signals but no brake lights. Has happened repeatedly. Now I don't use cruise control so I can have brake lights, BUT I need both. What's up with this? Any help would be appreciated.

Could be a short in one of the lamp sockets, or in the wiring.
The dash lights should be on a 5 amp fuse.

hot wire touching metal fuse blows so you don't burn up the wiring get it tested and fixed soon could be dangerous to keep putting in a new fuse

i had this problem with my 1999 jeep grand cherokee laredo with the brake lights it kept popping fuses right after the ignition was turned on try putting a higher amp fuse in place of the old one that keeps blowing then check for shorts to the starter and check grounds and wires and also the transmission issue check the fluid and then if the fluid is fine change the transmission filter the filter may have not gotten replaced when the transmission was rebuilt and may be clogged or dirty

check the switch on the brake pedal . do they work until you hit the brakes?

Here is an idea, when do you blow the fuse key on, key on engine running, key on engine running hitting the break pedal? Try and get the key on without blowing fuse if possible, then one at a time try those circuit loads ie: horn, brake pedal and so on, if you isolate which load is pulling to many amps(blowing fuse) unplug that load itself, then restart testing, see if fuse stays ok. You can also substitue the fuse with a circuit breaker and unplug each load until click(of breaker) stops.

i would look at the brake pedal sitch for chffed wires or shorted to ground

Check all the bulbs at tail end for parking and brakes, test grounds and harness for trailer. Check fuses. clean batery terminals, Re instal head light swich properly and make sure all wires conect.
Vitally

Check the fuse for your cigar lighter/horn. I've seen the fuse blown from the lighter shorting. It will caue the cruie not to work also. I hope this helps. Good luck.

There is a ground not connected or a wire open that is causing it to blow

Replace, or adjust your brakelight switch located on the brake pedal arm.