I was just chasing vacuum on my truck, and that looks like the heater vent box air solenoid line. On the passenger side is the one that opens the box, drivers side above the tunnel is the one that switches floor and defrost. See if its pulling vacuum at any time as the heat controls cycle.
Keith from NC - Vehicle addiction with an ever changing fleet!
2000 TJ - Daily driver and fun machine on 35's
1982 J-10 - Work in progress, but on the road!
Come and gone - 97 TJ, 95 YJ, 06 Commander, 98 Grand, 85 CJ
And a mix of other non Jeep stuff now and over the years for the family and fun!