blowing fuse on internal lights!

I am looking for advice re the 10amp fuse which covers the Internal Lights, Warning bleeps, Door Mirror operation/Adjustment and Central Locking. I had made a 30 mile journey delivering 4 rocking horses (I make them) to a customer and had to have the boot open so kept getting the 'door open' bleep everytime we stopped. However when done and about to return my door mirror switch did not move the mirrors out, then I realised that the adjustment did not work either, or the bleeps/int lights etc.

I called at Halfords and picked up a pack of two 10 amp fuses and a mixed pack just in case and quickly identified the blown fuse (listed as 'internal lights') and went to replace it but the new fuse blew as I inserted it. I disconnected the battery and put another new fuse in and on reconnecting it again blew, with only one left I tried again and again it blew. As soon as I went to insert I did hear the door mirror motor start before it blew.

Has anyone had this problem? and if so do you have a fix? or advice on what it could be.

Hoping someone may be able to help.

Allan