I presume these are outboard units that are mounted beneath the body. Either way, most systems charge in pretty much the same method, refrigerant canister is connected by correct hoses / fittings to the service port (usually low-pressure side) and the compressor simply sucks it in.
But knowing how much is too much is critical, as too much refrigerant can destroy a system. Depending on year of manufacture, it could be an R-12 system, which can be tricky, if not impossible to get. If it happens to be R-134a, you shouldn't have a problem. Most everything before the mid to late-90s used R-12 for vehicles, and R-22 for residential / commercial. Using the wrong refrigerant can create even more problems.
As to refrigerant type / capacity, there should be a service tag somewhere on the unit with at least a serial number, refrigerant type, and recommended refrigerant capacity.
Here's a link to an RAC Refrigeration Systems, which hopefully is the correct original manufacturer. Perhaps they can help if you can get a serial number off of the unit, or at least tell you where to find the information you need. There has to be some sort of ID tag or plate on that unit somewhere to aid in service.
Hope that helps...