Extra bulb hanging under dash by heater controls

[70ChevyCST]

Hi guys, I've been under the dash on my 70 Chevy CST-10 on and off for the past few weeks replacing windshield washer pump, windshield wiper motor, stereo and I noticed an extra bulb...(lights up when I ground it) on the same gray wire as the bulb for the heater controls. The wire is about about 5" longer than the heater control bulb but I cannot figure out where it went or where it might go. It couldn't be for the orig. radio...That had it's own bulb that came out when it did....can't be a glove box light because it needs metal to mount to to light it up. The glovebox is cardboard...plus there's no switch in line to enable/disable it when glove box door would open/close. I also thought that it might be for the ashtray...but there's no hole there for it to fit into either??...Any ideas where it might go?? Can't go anywhere too far away because like I said it's only about 5" longer than the heater controls bulb....Any ideas appreciated..Bob