thanks for your suggestions. I am willing to try anything at this point..
We have pretty much managed to potty train my 5 year old grandson, but he has some verbal ability which helps a lot. We used a video about potty training, lots of time on the potty chair (like you are doing) and reinforcers of M&Ms or another small candy when he "produced" in the potty. He got one M&M for going in the potty and another if his pull up was clean. Now that he understands what he needs to do and is willing to do it the only other thing we've done is take away video game time from him when he has an accident while playing. (Which is pretty much the only time he has accidents.) This took us almost a year to accomplish, so patience, patience, patience. ;)
Good luck!
I too have an 6 1/2 year old girl with autism. She developed normally until around 2 1/2 when after a series of hospitalization for asthma she started to loose her speech, started eating grass and dirt which she never did, stopped singing, talking, 'reading books' and drawing. She regressed and it was severe. Currently she is still non verbal and is wearing diapers.
I have noticed that she sorts of bends her legs when the diaper is dirty, we have noted the times when she defecates and we would put her on the toilet around those times. She has defecate in the toilet but she is still not used to it.
We are still using this method and like I said my daughter sorts of bends her knees when her diaper is wet - we are hoping she does it before. Try this method you might have success but remember every child is different what works for some might not for others.
Good luck