I drive a US plated vehicle and renew the registration by mail when necessary. I have a son in Texas and we drive up to visit him several times a year, so I need US ins coverage for those trips. My US ins carrier allows me to keep minimum coverage on the vehicle when in Mexico (the coverage is actually no good here, but it allows me to keep the policy in force) I send a note and let them know when I am headed for Texas and they activate the liablity portion, at an additional cost of course. Obviously, I keep Mexico coverage on it also, as it is driven daily down here.
The SD situation was driven originally, my opinion here, by residents of other states who saw a way around the higher sales taxes, personal property taxes, etc., in those states. It gained a foothold in Mexico almost as an afterthought when someone realized the ease by which one could maintain the US plates. It is actually a nice, though relatively small, cash machine for SD. There is no fine print involved, it is quite easy to maintain SD registration by somene who is not currently residing there, and an auto insurance policy is not a requirement to maintain the registration.
Having said all that, I agree with the posting above; the simplest, cheapest, most efficient way is to drive a Mexican plated vehicle, and my next vehicle will be plated here. I purchased my current vehicle new, before moving here, and I refuse to give it away for 50% of what I have invested in it, at least for another year or two.