GMC Repair: hooked up battery backwards, altinator, altenator

Question
hello tim my brother hooked up his battery backwards and his radio stopped workin brought it to the  mechanic (not that good but all we got) fixed some torched wires around the altenator and after that the speedometer stopped workin  i have checked everything out and so has the mechanic he even changed the sensor and the speedo and radio still don't work but everything else is fine
the speedo was workin before he rewired the altinator but don't see anything he could have messed up or forgot

Answer
hooking up a battery backwards can be a very bad thing, as the radio might be the least of your concerns, there is a chance that you fried every module in the vehicle, have seen it happen.  as for the radio its probably junk, if you were to take it apart you would see the circuit board is probably fried.  same for the cluster, electronic parts are made to receive power one way and ground another, when they are switched it gets very ugly