If the needle is pegged.
Reverse the meter wires like Steve says. That should take care of the polarity problem. However, if the needle is pegged aganst the stop, I can list some things to cause it (maybe not all things).
1) There really is that much current because something else is wrong. (least likely)
2) The meter is toast.
3) The meter and shunt are mismatched ( for example 60A meter and 30A shunt would cause 2x error). The shunt is designed to give full meter voltage at its max rating. The meter is designed to accept full meter voltage at max indication. Small shunt will cause high reading on large meter. It would take a fairly significant mismatch to peg the needle so this may not be it.
4) There is a short to aircraft frame ground in the "return" line between shunt and meter. This will cause full 12V at the meter terminals and "Peg" the needle. Since I am not an electrical wizard and didn't stay at a Holiday Inn Express last night, it escapes me how I would confidently check this with a multimeter. The brute force approach would be to fabricate a pair of external wires between the shunt and meter. Disconnect the old wires and use the new ones to debug. (EDIT - I think this can be proved by disconnecting the "ground side" meter line at the shunt. The meter should stop working. If it does not, there is a short to AC frame in that line)
5) Shunt is broken (this should be evident in other ways).
Hope this helps.
