Old furnace with extra wire, but no c terminal Janitrol, 19 year old furnace
Attach it to the junction of the fat blue wire and the skinny red wire

Since it appears like you have air conditioning, as the yellow wire from what looks to be your thermostat cable goes off to what looks like a connection to a white wire in a two-wire cable, likely running to the outdoor unit (compressor).

Because of this, we can make a good guess that the red wire from that two-wire cable connects to a C wire, and that guess is supported by the wire color scheme; the wiring diagram calls out the C wires as blue, and sure enough, the wire that red wire connects to is blue indeed.  So, I would try incorporating the C wire to your thermostat into that bundle, then.