If I tinted my tailights would it void my warranty?
It won't void your warranty, it will get you ticketed. Federal law prevents such tinting unless the car is used off-road or for show purposes only. This is for you safety. It is really dumb to make a car less visible at night, you want to be as visible as possible, or you will be rear-ended in a situation such as fog, rain, blowing snow, etc, when it can be almost impossible to see the vehicle ahead as it is, without having that vehicle's light output reduced. Look at any smoke covers and/or kits in the shops, they clearly are marked as "for show or off-road use only", for a reason.
I had new speaker and amp installed and now car won't start! Can this be caused by bad installation?
Oh these are always fun. Though I can't give you a diagnosis over email, I can tell you it is 100% possible to kill a car with a stereo installation. Your amplifier needs a signal to turn on or it will stay on always and kill your battery. Stereo install guys turn on the car and stab a test light into random wires looking for one powered only when the car is turned on. Unfortunately they have been known to splice into a CAN data bus. The CAN network in charge of communication for your security and locking could be disabled keeping your key from being authorized. Or the CAN network for your chassis systems can be shut down that deals with communication for your engine, transmission, and typically traction control. A CAN network is typically a 5 volt system but it suplies power in micro amps as it is only used for communication and not supplying a load such as switching a relay, lighting a bulb, or spinning a motor. Splicing into these to supply the relay to turn on the amp effectively grounds the circuit and does not allow for the digital signal to propogate. Sometimes it easier and they just blew a fuse while poking around with a test light, or shorted a harness by running a screw through it while mounting components. Had a lowjack installer put a screw through a floorboard and into a fuel tank once, cause a check engine light due to an emissions test, luckily it didn't burn down the car first. I have fixed at least a dozen of these problems on Benz's in the shop after a car got towed in from the stereo shop.