The little led light blinks orange and green which is the phones way of telling you its overheating. I monitor the temp with an app called battery widget. I think it starts overheating around 115 f my phone gets up to about 125 in my noted sitautions.
Those temps sound a bit high, assuming a "normal" surrounding temp, but it shouldn't be overheating yet...
Some things to try, since obviously a replacement is not an option...
1) Different battery, stock or Laza for standard, or an HTC extended battery. Note that the stock HTC extended battery is known to run about 15-30 degrees cooler across the board in most situations.
2) Different charger/cable (especially in the car, make sure it is capable of 1A minimum)
3) Factory reset
4) S-OFF and downgrade/upgrade
The last is most extreme and time consuming, but now that S-OFF is simple on the Rezound thanks to rumrunner, it isn't a big deal and there are step by step instructions.
Honestly, overheating is a major issue for this device... probably 20-30% of all defective ones were for this issue.