Passenger side heater vents not as hot as driver side

Picked up my 05 2.5i Legacy last month and thought I had checked the operation of the heating/ac system pretty well before purchase. However, I'm now noticing that the passenger side vents blow air about 75% cooler than the driver side vents do when the heater is on.

I do not have auto-climate control and the knob on the temperature control is turned all the way to the hottest it will go. The interior seems to heat up just fine, but it's annoying that the passenger vents still feel cold when the heater is on.

What could be the problem and is there a DIY solution? Car is no longer under warranty.

This has now occurred for me. Other reports indicate that its due to the heater core being blocked up.

A few related threads:

http://legacygt.com/forums/showthread.php/heater-stopped-working-101641.html?

http://legacygt.com/forums/showthread.php/heater-blower-issue-101270.html?

I only just did a coolant change using Peak Long Life coolant and the requisite 'Subaru coolant conditioner' which is really a stop leak product.

I think it contributed to it getting blocked up more than anything else.

My question is how to deal with this short of replacing the heater core which is $$$. I will try flushing the system once at home, perhaps try a local radiator shop and see what they say but I'd like to avoid having to rip out the dash and replacing the heater core if possible.