amp gauge

The shunt needs to be only a few inches long.  You could go to a hardware store and get a piece of solid ground wire, and some solder on ring terminals.  It would be better to not attach the shunt near the meter this time.  I am sure that the heat from the shunt is what toasted the original meter.  The 2 red wires that I see in the picture need to be connected to the shunt.  One wire to each end of the shunt.  You need to run 2 smaller wires from the same 2 junctions to the meter.  You will need to insert a 1K ohm potentiometer in series with one of the wires going to the meter.  This will be the adjustment to calibrate the meter.  You will need to figure out which of the 2 wires goes toward the positive connection of the tester.  That junction needs to go to the positive connection of the meter.

The pot is not that critical   just get the cheapest thing available.  We may have to get a different value.  The 1k is just to get the meter working, we may have to get a different value for actual accurate calibration.    If you are only dealing with 40 amps you could use an AGU 60 as your shunt.  Please get a Buss Brand silver fuse.  If you use the AGU fuse as a shunt you will need to get a good fuse holder.  Do not use a sealed one as sold by every Car Audio Accessory manufacturer.  They are sealed and the heat has no where to go.  Use one found on the bottom of this page.  Middle row of pictures second from bottom.  http://www.webnevesht.com/products/Fuses-Buss.aspx    The description reads Buss Bussman Single Pole Fuse Block BM6031PQ NEW $ 2.95

I know it is only rated at 30 amps. It is severly underrated.