You can get to them from underneath. Just use a rag and fish it inbehind the sensor and pull the cloth back and forth gently. You'd be surprised how much mud and grim builds up in there. I will be installing mud flaps to prevent more build up and broken wires from rocks shooting up in there.
That's perplexing. For one, the radius on that bend doesn't look right at all, i.e. there isn't enough slack between the connector and the post holding down the loom. Especially for wires that small of a gauge. Either there should have been a bit more of a bend radius on that loom, or Ford should have designed that connector to be a 90-degree connector to reduce strain on those hair-thin wires.
And yes, I've had mud flaps on since about the 4th week of ownership, the rear bumper cover on these act more like a snow plow than anything, scooping up any things rear tires/wheels kick up. The one that I had that almost really caught me, was a small piece of re-tread on the road flew up and luckily it impacted the spare tire first from what I could tell, I bet if it had hit the inside of the bumper cover it would have torn off and/or broken half a dozen sensors down there. Looks to me like you are adding some rear winter traction though with that sand bag worth of sand in there.