The GPS question is a tricky one. The answer is definitely yes if they get a warrant. The answer is very likely if they have probable cause, and a solid maybe even if they do not. Privacy rights do not exist in a car to the same extent as in a house.
As to your other question, probably not. I know the Lancaster County DA's office, and they by and large do NOT make deals on drug felonies. The best I know of anyone getting is an agreement to drop mandatory sentences, which still leaves the court plenty of power to lock you up.
Absolutely, however it is a lot easier and safer to just activate the GPS feature on your cell phone. Both usually require a warrant under state law. However after 9/11, the feds do not always need a warrant and the feds frequently give the State their information.
In New Jersey everything is negotiable. I have seen many defendants walk away from 1st degree charges with probation due to cooperation. However you must keep in mind that the prosecution does not accept every defendant as an informant. First, the prosecution is interested in “weight” thus, personal use quantities are usually not worth the time or money especially if you have a 1,000 foot case. Additionally a defendant may have great information but may be unreliable or unable to follow through on an investigation. Finally, the State does make efforts to protect your identify however it is usually easy to find out, thus safety also needs to factor into the equation. You need an attorney to present what you have to the prosecution.
Legal disclaimer: This answer does not create an attorney-client relationship and is based only on the limited information provided in the question.