Oliver;

There were variations, but if you mean the square, bystable relay, I believe that was mounted on the electrical panel with the fuseblocks. The fact that a fuse (Ignition power I presume?) is blowing suggests that there is indeed a short, possibly in the relay, possibly in the brights circuit, and activating the relay is switching it into the brights circuit causing the fuse to blow (tach losing power is an expected side-effect once fuse blows...as a matter of fact, ALL Ign powered load will be dead at that point!)

Ref to: http://www.intelab.com/swem/1800%20Wiring%20Diagram.jpg

...notice it shows a footswitch (fotomkoplare) and a standard relay for high-beam flashing, instead of the bystable relay, but I expect color code to still be correct.

Divide and conquer electrical problems...I'd remove the Blu/Wht (brights) wire, replace fuse, and try again, if fuse blows, short is at relay or associated wiring, if not, short is in high-beam circuit.

How grody does your fuseblock look...is it getting warm?...if so, read: http://www.intelab.com/swem/tech_bulletin_3.htm

Good Hunting!