Wire to am1 fuse burnt what gauge wire do I need to replace it on a 91 ls400

Find the fuse for the brake lights. The fuse size determines wire size

I believe that they are interchangeable up until 1997.

Check the harness bulkhead connector at the firewall. If you have been driving in deep water, you will get corrosion here. This connector is where power enters the cab (kickpanel fuse block). Use conductive grease at connector after cleaning the pins

fusible links are short bits of fuse and can be easily replaced
it should have burnt before the harness was damaged
they are available at the spares section

Could be a bad brake light switch or the solenoid itself or wiring. See if brake lights work. If they don't then check brake light fuse, if thats ok then check at the brake switch for power or jump the two wire plug lights should light and shifter work unless solenoid bad. Good luck

Do the gauges and warning lights work with key on? If not, check fusible links, AM1 for ignition key power. If the gauges/warning lights work, the key switch has power, so you can follow it to the starter solenoid. The start wire out of the ignition switch is a black/white wire, this is the hot wire to the solenoid, and only has power when key is held in start. With automatics, there is only one switch-a neutral start switch- between the ignition switch and the solenoid on the starter .

With a manual transmission, the black-white start wire goes to both a starter relay and a clutch start switch (on the clutch pedal), those two work together to send the power on to the starter solenoid. So, no crank, good battery, power to key, follow the black/white wire, the start relay and the clutch switch, to the starter solenoid- a hot wire there turns the starter motor on.

My first advice is to throw the out and buy something a little more dependent than that piece of s...junk. But if that option does not appeal to you for whatever reason try this.

Try replacing the fuse, sometimes the fuse may look fine but is not functioning properly. Or try replacing the relay switch for the wiper motor. If that does not work than you have a couple of different possible issues. I would suggest taking it into a shop to get it fixed because it will take time and skill to find the problem.
Take a multimeter and check the volts leading into the fuse. If you have no power leading to your fuse that the problem might be a wire the wire leading into your box.
If you have power there than you need to follow the wire leading out of the back of the box to your gauges. Check the power at each point in the sire system leading to your gauges and you will know if a stretch of wire is bad.
If one part of the wire is burnt or fired than you will need to check the volts and amps leading to your wire and find out why you are getting too much power to that area or it will continue to happen until you fix the issue.
If this does not help you than i would need to take a look at the car to help you more.

There are at least two, and maybe three Fusible Links just off the battery on white wires. You probably blew them out. They can be cut out and replaced. One is a 60 amp fuse link, designated AM1-it is for power to the ignition switch. Another fuse link will go to the alternator. You need to check them all.
A parts store can probably find the correct replacements. If not, go to the dealer's parts place.
When a fusible link blows, the wire insulation may be partially melted, the wire may look and feel like it has bulges. They are about 3 inches long. You cut the wire before and after them, and solder in a new fuse link. Take the old bad ones with you to get the proper replacement.

check for a blown fuse in the fuse panel and if its not that pull the gauge cluster out of the dash and check the power and ground going to it cause most of the time its a bad ground wire or power wire cause people tend to kick wires under the dash on accident or they get worn or come apart

bad computer. had the same problem sent mine to auto computer exchange in Davie Fl. they reprogramed it . The problem is gone.