Auxiliary brake lights are inexpensive and easy to wire in. Regardless of legal issues, it's just NOT safe to have no brake lights.. I happened to be behind a guy with a bike carrier, with 4 bikes on it. COMPLETELY obscured his rear end.. Couldn't see his license plate, tail lights, brake lights, NOTHING..and it was on a Jetta, his entire rear view was obscured.... He happened to pull into the gas station I was going to, and I complimented him on his bikes (Trek's, I've had a couple).... then started asking him about his setup and politely remarking how unsafe it was. The guy flat out said he LIKED IT THAT WAY, because people kept their distance, and he mistakenly thought that if someone DID rear end him that he'd get a huge payout to replace his cars/bike.. I pointed out the fact that 1. Obscured brake lights are against the law, and if someone rear ended him, at BEST it'd be shared fault, but most likely ruled HIS fault, and 2. Obscured license plate = illegal (he commented that he didn't have to worry about red light cams, half joking half serious), and 3. it endangers his family, regardless of whose fault it was..... He politely dismissed me.... We agreed to disagree and parted ways... The only good thing to come out of it was seeing him get pulled over a block later for having an obstructed tag (Gotta love having a county portable).
But, that just goes to show you, as mentioned earlier, the 'I'm invincible, it won't happen to me' attitude. I have a hard enough time with vehicles tailgating, not paying attention and almost hitting me, etc as it is, I don't need to HELP THEM rear end me..
If you DO mount stuff that blocks your factory lights, the very LEAST you can do is mount add'l lights that aren't obstructed... It takes 2 seconds to add lights to those racks you slide into your hitch, and a lot of them are available with them preinstalled.