I would agree, a harness would get in the way with the limited space inside the narrow body.
Here's a TCS 21 pin without a harness, should do the job.
http://www.dccsupplies.com/shop/product_info.php?products_id=4721
I recently picked up another 141 and decided to just put a non-sound decoder in it. (My other 3 141s are all DCC sound chipped.)
I ended up using a Hornby Sapphire that I had lying around, works fine.More expensive than the Bachmann, but I think it has a few
extra programming features.