On a 99.5 Jetta (MK4), you need to pull the dash and disconnect the a/c lines to the heater/ac climate control box. Some people are able to remove the heater core without having to completely remove the box loose from the vehicle, thus saving time. It is not a job for the faint of heart, nor is it a job for the average do it your selfer--- The original Soficia? heater cores weren't great and the Valeo cores are better. I have been told that at least one company makes a heater core out of copper instead of the alloy and plastic OEM core that was installed at the factory, but I've never seen one of these copper heater cores. ( I have seen a copper radiator replacement for the alloy radiators, so I have reason to believe that a copper heater core does exist).
Your least expensive choice would be either a VW specialist or an a/c specialist to replace the heater core. My suggestion is either a dealership or the independent VW specialist replace the heater core because I've seen too many jobs that have been done poorly and have either damaged the climate control box or failed to install the box assembly correctly (they can rattle badly if not secured properly and its a pain to fix) or failed to adjust or connect something.
I hope this helps, a car nut.
How difficult is it to change the heater core on a 1999 Jetta?
This takes a long long time and is no piece of cake. Give yourself 10 hours to do this to unfasten and refasten every nut, bolt, and screw on there. The late model Jettas are not known to have good heater cores and they are infamous for this problem.
