No gas gauge

[KGBJ]

Hello,

I've got a 1985 Oldsmobile 98 Regency Brougham. The gas gauge hasn't worked since I bought the car a year ago, and I've recently got around to trying to fix it. The factory service manual states the illumination behind the gauge will not work if there is a problem with the circuit, so I took my gauge cluster out to test it this weekend, and followed the diagnostic chart. First I checked the ohmic resistance in the circuit for the dash wiring and it was in spec (27.4 ohms), the manual said between 2-40 ohms is in spec. Then, I followed the next step, check continuity on the printed circuit of the instrument cluster. It had continuity, so as per the manual, replace the instrument cluster. I swapped all my gauges with an instrument cluster I got out of a 1988 Olds Delta 88, and even tried swapping the fuel gauges. The needle set itself to full when I turned the car on (with a full tank) but there is still no illumination behind it and it is not going down as I drive. Any ideas as to what could be causing this, when the gauge out of the donor car (and possibly the original) are good? There's little chance both clusters are bad, and I'm sick and tired of trying to count my miles to find out when to fill up. Thanks!