Fireman Tha Carter II Lil Wayne

so heavy in the trunk, made the car pop a wheelie

Probably refering to how heavy his subwoofers are, which are usually placed in the trunk. (His car's sound system has a lot of bass)
Hes more than likely referring to having so much drugs in his trunk that his car is heavy in the rear and lifting the front end up. Hence, popping a wheelie.