What is the car doing to make you believe the pcm's are needing to be replaced? How come they are getting wet? They are fairly protected in the stock location. Headers will burn through spark plug wires if the wires are allowed to come into contact with them. I have a 98 also and what I did was get some heat resistant tubing and make a sheath for each plug wire. These have been on the car now for about the last 3 years and I have had no problems with heat affecting them.
94 Formula A4, 1 of 9,225 SOLD but not forgotten.
98 V6 Bird
00 Formula A4