My daughter blew her heater core during the rush going South on 101 just pass Blossom Hill. You know what thats like the Friday evening before Christmas.
I couldn't tell for sure if it was a supply tube or the heater core. But there wasn't much room to see where the damage was. I decided to pull the heater. I picked up a Haynes manual for the 84 to 88 190E (hers is a 90 2.6) but the manual was close enough to get me through it all. I wish I could remember back that far (going on two years) so I could tell you if the tubes come out on their own...I would think that if you can get the bolts out, that may be half of the battle. But there could be some wierd crooks in them that would make it though.
If you have to yank the dash, it's not really that big a deal. Just a million things to remove, air bag, steering wheel, trim on the A pillars, speakers, instrument panel, glove box, center dash switches, and then maybe the dash. While your in there you may want to consider changing the vacuum elements. Make some notes and drawings. The Haynes manual will walk you through the whole ordeal.