Where can i find 12 volt 10 to 15 amps source under the dash

the fuel pump is controlled by a fuel pump relay

I installed a fuel tank in the bed of the truck with a small fuel pump. No I need to find 12 V somewhere underneath the dash to run my auxiliary fuel pump

Check the fuse box for something that runs on that amperage and tie in a fused switched or actually looks like many fuse sockets are not used so you can create your own with whatever amp fuse you need wired into the fuse box look at this .....https://www.youtube.com/watch?v=jXhtBYM-QDY .... keep in mind the fuse is only the 12 power and a switch will need to be wired in also the pump must be grounded.

tying in to an existing fuse that operates something is not always a good idea so I would create my own.

Use a piggy back fuse to power the transfer pump.