If your mechanic told you the dash has to come out to replace that heater core then you need to find a more qualified mechanic. Aside from the fact that you need to be a bit of a contortionist to maneuver under the dash, it is really not very difficult to replace that heater core and does not even require removal or evacuation of the A/C evaporator. Disconnect the negative battery cable, drain the coolant, and disconnect the heater hoses under the hood. Then remove the right side lower sound insulator panel and the glovebox, remove the BCM/ECM mounting bracket, disconnect the connectors to the BCM/ECM and remove the BCM/ECM, disconnect the rod from the damper door programmer, disconnect the wiring connectors and vacuum hoses to the programmer, remove the programmer, remove the heater core cover, and then remove the heater core. And as for how much you should expect the job to cost, figure the cost of the heater core plus coolant (heater hoses if necessary) and labor. The labor time guide for the 1998-2004 Cadillac Seville STS and SLS list 2.4 hours labor. I expect earlier and more recent models will be similar.
