Hyundai Matrix heater problem

It's a 2001, came in with suspected air con problem (not cooling). But ac system is working, expansion valve block at back of engine bay is chilled.
Ambient is about 15 degrees c, temp from vents with heater set fully cold is about 20. So, it looks like the heater is stuck on a bit. I found the heater control servo on the left of the air box, removed this and manually pushed the controls to fully cold - still it heats +5c with the ac cold.
Any ideas?
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