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Turbo Sd Charger
You'll need to look at the cost of the new engine (sorry can't help you with that, try calling around to shops in your area), look at the worth of the vehicle before and after replacement of the engine, and how much a "new" car will cost, then weigh your options.
This may be tires, especially if you just put new ones on and this started after that. Take it back to where you got the tires and they should look into this for you, they may need to re-balance the tires again.
Try wrecking yards, and craigslist.org
This info might help: http://repairpal.com/OBD-II-Code-P0300
If this is happening when it's not raining, then the wiper leak is possible, as well as a possible plugged A/C evaporator drain that is causing a water leak up under the dash, and the water is running to the back of the vehicle. Has it been raining before you have the wet carpet? Have you been using the wipers/washer in the rear? What about the A/C?
They rebuilt the transmission and didn't do anything to the converter? Did it have the same problem before and after the rebuild? Are they requesting that you pay to have them replace the torque converter now after the rebuild?
There are little motors(actuators) inside the dashboard that operate doors that direct the airflow and temperature in the cabin. One of these actuators or flaps is not working properly, or the system that controls the actuators is having a problem. You will need to dig into the dash to see if these actuators are operating, and see that the control module is working properly.
This could be several things, sounds like a fuel pump, but that is a pretty wild guess. I would get it to a shop while it is having the problem so they can diagnose it.
This should go back to the shop that performed the work, this may be indicating a serious problem, so get it looked at very soon!
This site will sometimes have a parts picture that might help: http://www.gmpartsdirect.com/catalog/frameset.cfm