1998 Cadillac Caterra has a strange smell, especially when heater first warms up?

I have a 1998 Cadillac Caterra and it always has a smell that is kind of like something is leaking onto a hot engine part. It is coming from the front penger side wheel well area, and it is especially strong when the defrost/heat first warms up. If we shut off the penger side heat control, the smell diminishes greatly. The oil levels are maintaining themselves properly as is the coolant level on the resevoir. Any ideas? I have cleaned the crankcase, the crankcase filter, added seafoam, and many other things and cannot get the smell to stop. The engine does not show any signs of overheat either on the gauge or in breakdown of the oil and coolant.