I've never unclogged these drain lines, and I actually have never looked at where they come out underneath the car, but I did replace them when I replaced the evaporator on my W210.

There is one on each side of the center console. IIRC, if you remove those PITA panels underneath both sides of the instrument panel you should be able to peel back the carpet at the center console and reveal the pipes (flexible, insulated, spongy thingies about 6 inches long). They are clamped to the bottom of the box that holds the evaporator and heater core.

If you try to snake these out from underneath the vehicle make sure that you are not poking too far into that air handler box - you don't want to hit the evaporator.

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1996 C280 289K Traded
1997 E420 167K Traded
2001 S430 240K Traded
2010 E550 4matic 80K
2000 GMC Jimmy 198K Gone to Boneyard
2003 Camry LE 196K
2011 Mazda3 i Sport 31K