I have found from my personal experience with blazers and most other GM products that its most convienient to buy an assortment of differnt size gromets from a local auto parts store and drill a hole through the firewall below and to the left of the brake booster(if looking from under the hood). I personally use several reference points, both under the dash and under the hood to locate a place that will be free of wires and such. Let me know if i can offer any more assistance.