Well out on our night ride at Truckhaven around 1am I lost power to my GPS = lowrance 540c baja.

In checking it, the fuse was out, I replace it and it blew again.
check power line it's OK try and run new line with fuse to unit and another fuse blew
unplug line from unit and power good put new fuse and try to plug unit and blew fuse again

So it look like it's inside the unit, has anyone had this problem and can you go inside the unit or does anyone know how to fix it ;-(

Unplug the antenna and try it. There is power going to the antenna as well. Be sure to check that run- not sure how it is fused, night share the same main fuse.

I unplug the power to the unit and the fuse did not blow ;-)
I only have one 20a main fuse for both.

Without checking the book I think we used a 10A main fuse, but put in an in-line 3A fuse as required.

I would also check your antenna and cable. Power does run through that and could be your culprit as stated earlier.