No high beams, no horn-Try this fix:
 

I could not get the high beams to come on or the horn to work. I could hear the relays clicking but no power was going to the lights or horn. While testing the fuse connections with a voltmeter I was able to get the high beams to operate by making a connection between the fuse and its connection. I cleaned the connection and installed new fuses. It worked some of the time. I decided to cut out the fuse box and replace it with a generic 5 block unit with the new plastic type fuses. The lights and high beams work as they should and appear brighter. The horn is also working. I temporarily mounted the fuse block within the original holder with assorted washers and nuts. It looks okay but doesn't look original if you are trying to keep your car that way. My XJS is an 86' and had the single headlamp fuse box located under the hood on the left side.

Good work.

Usually the original fuse boxes are brought back to life with a good clean-n-tighten-the-terminals routine but, if that fails.....


Cheers
DD

I went the route of cleaning and tightening but still had issues. This fix isn't for someone trying to keep it original. I made a decision to upgrade components where I can for better reliability at the expense of keeping it original. I am going to do some minor fab work to make it look like it should.

Under bonnet (hood) fuse box
 

FYI in case of need, interestingly, on this age of XJS (mid to late 1980s) the fuse nearest the front of the car controls the auxiliary electric radiator fan, AND horns, but none of the wiring diagrams show that the fuse controls the feed to the horns. The diagrams just show the fuse controlling the auxiliary fan.

Greg