Factory gauge dash on a '68 Caprice (I know, but the folks at Chevytalk.com ain't as friendly as we are), Needle stuck between discharge and the middle, wiggles if bumped but won't respond. I see no info in the service manual at all, and don't know where to start to diagnose this problem. All advice welcome!

Never worked on a Caprice but they should be similar to a Chevelle. With the car not running and headlights and other accessories turned on the gauge should move towards the discharge side. When the car is first started, the gauge should move towards the positive side for a little bit, then move close to the center towards the positive as the battery becomes fully charged. The amp gauge will only stay far towards the positive if the battery's really low with the car running. The amp gauge has 2 connections, one connects to the junction block near the battery, the other connects near the 4 way splice or horn relay, see pic
http://www.madelectrical.com/images/chevym1.jpg
There should be fusible links on both connections for the amp gauge. You would need to check these and use a ohm meter to check conductivity of the circuit. It is possible the gauge is damaged too. If the fusible link for the fusebox ever blew, power would try to go around it through the amp gauge, and the fusible links for the amp gauge were known to be a little strong for the amp gauge circuit.

THANK YOU!! I will check this weekend and report back.

Dave

You can easily test the amp gauge circuit by lifting the amp gauge lead at the J-block. Reconnect the other two wires, power from the battery and power to the horn relay, snug the nut. Check for voltage on the lifted lead. If you have ≈ battery voltage it means you have a complete circuit i.e. no open amp gauge or fusible links etc. If it is a complete circuit you can use a 1.5 V battery, D, C, AA etc. to test the gauge. Hold on end of the battery on the J-block terminal and, with someone watching the gauge, touch the amp gauge lead to the other end of the battery. With the battery + held on the J-block the gauge should indicate charge, with the battery ─ held on the J-block the gauge should indicated discharge.

Note: A new D cell battery will peg the gauge. If you use an AA or AAA battery it will supply less current and probably won’t peg the gauge. Also you may want to disconnect the battery ─ terminal while wrenching on the J-block. You can leave it disconnected just referance the volt meter to the battery ─ terminal when checking for voltage on the lifted lead.

I have an Amp gauge that is not connected on my '71. This is just part of an aftermarket cluster added under the dash. Is an ampmeter or a voltmeter more useful?

A voltmeter for sure! The aftermarket amp meters reqiured heavy wiring all all power to go through the gauge, read this link on the subject
http://www.autometer.com/tech_faq_answer.aspx?sid=1&qid=5