Manual Heater Hose Valve

Home Depot sells a 1/2 ball valve. I have one on my car at the firewall and shortened the open/close handle. $8 and works great. The few times a year it gets chilly I pop the hood and open the valve.

Actually a 3/8 ball valve and 3/8 pipe nipples are the perfect size for 5/8 heater hose