Ok, I'm going to go out on a limb here, as I've never installed either of these Eclipse units. Assuming the problems were with the same vehicle, did you use the same harness from the 5510 as with the 6620?
Who installed this unit? Since this is happening with two different units, I think we can assume it is not an equipment malfunction. You didn't mention what vehicle it is mounted in, but it really almost sounds like you've got the tach wire connected to the steering wheel input. Just a guess here.
The first thing I would do, is ensure the power connections are correct (Battery, Accy, and most important, a good ground). After doing this, ensure your speaker connections are correct. This really won't have much, if any, affect on the problem, but if one of these wires are connected incorrectly, more than likely there is another wire that is also connected incorrectly.
Once these are verified, then you can move on to the other wires. Verify your illumination input, reverse, Speed sense, parking brake, & whatever else is there. If need be, cut the wires and start again. It would be a good idea to even connect one wire at a time to see if you can find the one wire causing this problem (or cut and re-splice one at a time).
EDIT: Sorry, just noticed in your sig it's a T4R Limited with steering wheels connected.
Since you do have these connected, try disconnecting this connector with engine running & while having the problem to see if it stops immediately. Then press on with the other wires if it doesn't stop.
