Fender well battery

I have been driving limos since 1978, and have had several different ones, both "factory" Cadillacs, and "after-market stretch" Lincolns. I'm not sure what you mean by "fender well battery," but many stretch limos have an extra battery, either under the hood, or in the trunk, which can be "kicked in" by holding a push-button switch. This can be used to start the car while holding on the push-button switch, in case your primary battery has been drained from all the extra doo-dads that are added when the limo is built, such as the TV/VCR, stereo, etc.