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Re-checking the balancing would be a cheap starting point, along with a close inspection of the wheels, as UAART1 pointed out. If you can locate the corner where the vibration comes from, exchange the wheels/tires positions around and see if the vibration source changes or follows a specific wheel.
After that, shaking under acceleration could be a CV joint.
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I would bet the liquid was most likely absorbed by the carpet and/or underlayment (or sound deadener ... whatever you want to consider it). Some may have gone out the holes as you are thinking, but I think I'd be testing the carpet in this area to see if it feels wet or sticky. If so, use a carpet cleaning machine on these areas to get the residue out. I ...
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Well, it is possible that the aftermarket radio messed up the CANBUS in the car somehow. Because your car is fairly recent ~2008, the CANBUS allows the car's immobilizer to communicate with the ECM. The backup camera could've messed up the CANBUS, as the CANBUS network is very susceptible to interference from other electrical system. Try rerouting the backup ...
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