Usually they do not search you when you go into mexico. However at the border they do have chckpoints just like the US. Usually you will pull upi and there will be a stoplight. if it turns green you are allowed to pass with no problems. if it turns rd they make you pull over and they look in your trunk and ask you a few questions.
As far as
syringes go, if they are filled with gear already and the mexican customs finds it, I really don't know what they would do. Just t be safe I would hide them in a well hidden place inside your car. Just be smart and don't look nervous if you get asked to pull over.
Maverick