What happens if someone who doesn't like you managed to unlock your car and plant drugs in there?

It will depend on your state but the ONLY defense you MIGHT have is if you request a hair follicle test to be done on you for drugs simply because that specific test can go back years to test for drugs and if that comes back clean you have a SOLID legal argument because depending on the drug(s) in question some drugs will test negative in as little as 24 hours while others can linger for a month or more!

Opening a locked car is actually quite easy for just about anyone. Doing so without setting off an alarm is more difficult.

Not even a MORON would believe your claim., because not even a MORON would waste valuable drugs in that manner.

A blood test is 100% IRRELEVANT to a charge of possession with the intent to distribute. If someone went to that much trouble to frame you, they would certainly plant enough to rate that charge.

They don't have to believe you, drug tests don't lie.  If you are truly innocent, you will be released after the results come back clean.  Then you can go about charging the person responsible for the crime, he or she will be guilty of possession of illegal substances as well as committing other infractions (breaking and entering,etc).  However, if it is found that your prints are on these items, you may be facing charges for possession if an actual suspect doesn't materialize. 

That defense has been tried before - it doesn't work.