RepairPal Certified shops can diagnose and repair your 1998 Mercury Mountaineer. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
I will assume you mean recharge the AC system. From the air compressor you will see two metal lines coming out. A thinner one and a wider one. The wider one is the low-side (lower pressure side) Follow the wider one till you find the schrader valve.