I just did one in my ex 1991 Jetta, yes the dash had to come out but it's not really that big of a deal. Just time consuming. (FWIW the reason it failed was probably green coolant)

How much is a new heater core? If its cheap you may as well do it while you have the dash out.

-Jason

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.