…I am going to assume being you stated bypass, that your heater core was bad and that the previous owner bypassed it rather than changing it.
you can definitely do it yourself if you have common knowledge of vehicles and common sense.
I just did a thorough writeup on how to do it; I will post it here.
the part is about 40.00 on rockauto writeup for heatercore replacement. with pics.
1. disconnect negative battery terminal.
2. partially drain cooling system
3. loosen hoseclamps to heatercore(located at firewall behind the right front strut tower) remove hoses
4. open glove compartment and open door at rear of compartment, then just remove the clip from the compartment. no need to unplug the plug. it is SIR
5. remove the 6 torx screws (4 facing forward on each side and 2 inside the compartment facing downward)
6. pull compartment insert out with door open and set to toward the center of car. (Be careful as there are 3 plugs on the left side)
7. mark thread on the actuator lever on top of the heaterbox, then remove it by pulling upward on the threated lever (note on the Seville, Eldorado, and console shift Deville there will be 2 actuators, due to the dual zone climate control)
8.unclip programmer from the firewall at the right front corner of the dash by releasing the clip and pulling the unit forward and out. Let it lay on the floor (no need to disconnect it)
9.look at the heaterbox and you will see 2 5.5 mm brass screws, (cant miss them) remove them both and the case splits revealing the heatercore inside.
10. inside the heaterbox there is 2 more brass screws that retain the holding bracket for the heatercore. remove them.
Installation is reverse of removal, but one important step before you reinstall the glove compartment. Reconnect the Actuator linkage(s) and reconnect the negative battery terminal.
Turn the key on but dont staart the engine. Watch the actuator(s) do 2 full cycles and come to rest. Then test the actuators by raising and lowering the HVAC temps. Once you have verified all is good reinstall the glove compartment.
Reconnect heater hoses, fill and purge the system.
Total job took me 32 minutes. one of the most decent heatercores I have done.