Wiring problems......driving me mad.

Good evening.
Have just noticed that when you press the brake pedal on my 110 that the rear tail light goes but the brake light does not illuminate. Checked the fuse box and found fuse number 4 blown. Fuse 4 also has the reversing lights on it. It an ex military 110. Without using the brake pedal the tail lights stay on as normal.

Have checked all the usual suspects, bulbs, earths, replaced bulb holders. Have checked with a meter and there is power down both wires to the brake/tail lights.

Replaced the fuse, started the car and the fuse blew again immediately.
As the fuse is also linked to the reversing lights I disconnected the two wires from the reversing light switch and tried again....fuse blew straight away.

I even bypassed the brake light switch thinking it may be faulty but again the fuse blows as soon as you turn the key.

As the fuse blows as soon as you start the car, before you even touch the brake pedal should I be looking for a fault between ignition switch and fuse box.

Could problems the reversing lights cause the fuse to blow.?

Any thing I have missed?

Kind regards
Mark