Heater core leaks are common in this model, more so if the wrong anti-freeze/coolant is used in the cooling system.
Car Problem Reports
Volkswagen Jetta Wrong Anti-Freeze/Coolant May Cause Heater Core Leaks
Volkswagen Jetta Problem
Average Mileage: 141,625 mi (101,250 mi - 182,000 mi)
Visitor, 1997 Volkswagen Jetta
Heater core failed in 2008. Began leaking when A/C was on quite badly in 2010. Heater failed completely in 2011. Didn't have the $500 which covers the labor (independent garage not VW) to pull the entire dashboard to replace the heater core. Bypassed heater in the meantime.