There are two of you? And the legal status of your vehicle (does not need to be answered if you don't want to advertise it publicly!)? I ask regarding the vehicle if that too needed to renew a 180 day visa.
Are you opposed to taking the bus? Mexican bus service is two steps above typical "Novelo's" service and Belice City is now served by ADO, the big and much better than Greyhound dominate bus carrier. I have a friend taking the ADO bus from Playa next week to Belice City. I didn't see the bus listed on the ADO ticket site, but this link quotes Jan '13 price of $496 pesos. http://www.islandexp...ize-city-cancun
ADO service between Playa and Chetumal costs about $300 pesos. A taxi to the border might run $50 pesos. Did you only want to go to Corazol? In order to renew your visa for another 180 days, Mexico requires that you exit the country. There has never been any requirement that you be outside Mexico for any length of time.
What I encountered in Belize a handful of years ago was the most corrupt and threatening situation by government officials I have ever experienced. That is a pretty strong statement if you knew my history and adventures in areas with strong travel warnings issued by both the U.S. and the host country.
I needed to see someone in Belize City, fortunately briefly. Upon entering Belize I was asked how long I intended to be in their country. I said a short time, I'd spend the night in Corazol and then take the bus in the morning for a lunch appointment I had around noon. Customs stated that I would only need then three days? Yes, I replied, "if it goes smoothly, I'll probably return tomorrow". No problem, three day visa.
I lucked out, completed my errands and by chance returned just in time to catch the last bus that was going all the way from Belize City to Chetumal. The whole affair was a set up by Belizean Customs hoping for a chance at extortion upon my return.
I didn't know, but I probably might have gotten away with skipping the exit stop at the Belizean Customs. Turning in my visa I was hit with, "but this visa is for three days, you have only been here for 24 hours". Yes, I accomplished my mission and now I am returning. "No, you need to stay two more nights". "We're tired of people exiting Mexico just in order to get a new visa". I was confronted by three uniformed men, armed, two of which were fingering their holstered side arms. Oddly, these were the only three "Anglo looking" men I had seen and the only ones that spoke English fluently without any accent. They looked at my passport and stated that the three day visa I had been given the day before was from noting that they could see that I had been living in Mexico over the years for extended periods. On that particular visa which I had surrendered to Mexico, it did have two months left and I did intend to remain for another three months. My purpose of the trip was not for a visa, but I did have that two fold accomplishment in the back of my mind. In a threatening standoff that they were demanding $300 USD (while rubbing their gun holsters) I finally pulled out three $20 dollar bills and put them on the counter and said that's all you get. I'm leaving now. I had considered making a run for the border and I think they would have shot me in the back.
If I was to go again, I'd be tempted to just talk with Mexican officials and then go into the Free Trade Zone for lunch and come back with plenty of time to catch the ADO bus that would give me enough time to catch the last ferry back to Cozumel. If you aren't experienced with Mexican bus service, you really should try it. It would be so much easier in so many ways and probably far cheaper too.
My opinion of Belize is poor, I think it was of major benefit to Guatemala to be rid of their "Eastern Province". There is no way I would consider being responsible for a rental car. Bus service is so easier and so much safer, economical too.