The remote sensor on my 990 is blocked for now. I'm using a Harmony 880. What infrared sensor can I hook up to the input on the 990 so the Harmony 880 will control it? Thanks in advance.

I use a Xantech system. Ordered off http://www.enhancedht.com

Be aware that if you intend to use the rear IR input that you may need an interface module (for Xantech it's a 794-10). If that's too pricey for you, an IR repeater will also work just fine.