What is the Small Red Vac line on Passenger Side 89’ 4.0

[oleskool]

Like the title says. What is this vac line for? I am assuming it’s a vac line. I circled it and where it goes on the engine. It runs into the firewall. TIA 

[jdog]

The heater switch,if possible you can call the end on the firewall and delete the heater hose piece

[tugboat95]

The heater switch,if possible you can call the end on the firewall and delete the heater hose piece

Yep, I capped mine off when I did the open cooling conversion. And just cut all that out.

1989 Eliminator
4.0L 5 speed
2wd

[cruiser54]

Heater control valve. Eliminate it and cap off the line.