My 2 cts, the plastic scuttle panel is sealed against the windscreen with butyl tape that with time and heat etc gets dry and does not seal well, not a problem you would think, but you will find that with heavy rain some water will seep through the gap and end up dripping over the heater motor and get in and if the seal between the wiper motor and the firewall is not good (and it probably won´t be after 20 years) you will get water inside the cabin that way too despite the drains being unblocked under heavy rain if you park outside, I resealed mine with new butyl tape after cleaning and it´s been perfect since. If you want to go all in you can also replace the seal between the heater fan and the firewall but that involves taking the dash out etc.