Have a short in the Horn/dome light fuse on a 1990 chevy suburban, keeps blowing fuse even when truck isn't on. using a 20 amp fuse and with truck completely off as soon as the fuse is put in it blows it. i've taken the contact for the horn off and took the dome light off and still blows the fuse on contact.

blowing fuse 4 ,F4, 20 amps? (not trailer fuse,marker)
it feeds the brown wire out of head/park switch
13 park lamps on BROWN

well the park feed is shorted clear as day.
only where it shorted matters
park lamps can be 12 lamps.
2 front, 2 read, side lamps
license plate lamp, rear
the seam park feed goes to the cab too.
seems dome is shorted
and lamps sure can short out and the wires
this old easy.
the online manual sticks no dome ckt shown
old trucks then had no digital manual
so all we see today are lamp weak and grossly incomplete scans.
buy a used real F150 manual off ebay OEM real.

What is the amp rating on the fuse that keeps blowing? I looked at wiring for stop lamps, 15 amp fuse. Unplug connector for stop lamp switch, see if the fuse still blows? Also see if anything at connector looks burned or melted.

You could have problem with fuse box, itself. Look at back side of fuse box, see if anything looks burned or melted.

Not easy to diagnose. Most probable cause is an intermittent short in the power wiring between the hazard module under the dash and the brake and center stop lights, or a problem with either that module or brake light switch on the pedal. I would start by inspecting the socket wiring and interior of the sockets on the rear light modules.

Sounds like you need help

Have a repair shop resolve

When you start putting larger fuses into any
electrical device, your not thinking clearly & need
to stop & have some else resolve things

there might be corrosion on the ends of the fuse holder check that or try tuner cleaner before inserting the fuse.could also be a short in the fuse holder.

Why would you replace a fuse with the wrong amperage?

I will at least give you credit for knowing that was wrong
& could in some cases over heat & burn vehicle wiring

What was that going to do or prove?

Surely even with absolutely no vehicle knowledge
what so ever, you would would realize that you need
to have a shop check out or test the truck for short to ground,
poor grounding etc on the dome light & what ever gauge lighting
circuit issues you have

What is really sad, is the truck isn't very old & you want to
risk further issues with the fuse deal

could be you might check it out. many items take the same relay so you might try exchanging it with another one with the same numbers. If that isn't it you may have a short in the horn itself.

Check inside the drivers and passenger side door jam for the little push in type switch. 9x out of 10 one or the other will be causing the short to the dome fuse. These little **** get rusty and also get alot of wear and tear from the door opening and shutting over and over. These switchs are what causes your dome light to come on when you open your door and or likewise they shut your dome light off when you shut you door. A very good way to test to see if one of these little switchs is the couprate is to take and hook up a voltage light into the negative side battery post after you disconnect the battery negative line.. If your voltage light is on than that would mean that you have a short somewhere in your system. Now, since you already know that you are blowing the dome fuse then your already halfway there. With the light on your votage light still on indicatin a short, unscrew the little switch on the door jam and disconnect the power wire from it. Check to see if the light on your voltage light is still indicating a short, if not then you can be sure that the door jam was the coulprete. Hope this is of help.

horn and dome on same circuit,i would check dome wire under carpet

check your cab lites to see if water has gotten into one of them and corroded a hot wire off. and that it's not shorting out causing your problem. also take a look at the wiring going to your domelite.