Could be a bad auxiliary pump. Bottom left side (driver side) of the radiator.
Usually when this one goes out, there's no heat in the cabin.
To check: Open the expansion tank cap, car in "P", key in pos2, HVAC set to your normal heat comfort.
You should hear the coolant gurgling, and also see a stream of coolant (if it's topped off) coming from the upper snaked hose that sits on top of the radiator. If nothing happens, the pump is dead and needs replacing.
Looking for a DIY? Parts? Check this out, it might be your ticket
TMS underdrive pullies - Stewart WP - PSS9 - Beisan Vanos seals - Zimmerman cross-drilled & Akebono Euro - Deka 649 MF - 55w HID headlights - 35w HID foglights - Hualigan double din - ACS (rep) alu pedals - Euro central storage console - Breyton Magic Racing staggered wheels - M5 bumper - M5 steering wheel - Tint
Stable: e39, e53, e46 & Tribby