On the heater or heater/AC controls you have 4 light bulbs. 2 above and either 2 below or 1 below and 1 on side depending on year and heater or AC/Heat.
For the speed you have the main RH and LH bulbs. There is another bulb at the bottom at the shift quadrant and the high beam bulb. Look at Fig 15-3 in your shop manual for an illustration. here is a scan form the 1964 Shop Manual which happens to be the same Fig #:
My left side of speedometer used to be much darker as well. I turns out the little speedometer light bulb wasn't making a great connection. I'd go under the dash with the lights on and turn it slightly (or reinstall it) and I bet you'll get it going again. That's what I did and now my speedometer is lit up like a Christmas tree.
Look at your shop manual page 12-95 Fig 12-109 in lower right corner. It shows two lights for "Heater & Vent Control Dial - Lower" and then below it two lights for "Heater & Vent Control Dial - Upper" for a total of 4. The 64's had 4 bulbs, but you had 2 on top, one on bottom, and one on the far left driver side next to the temp dial.
Replace with LED you will be happy