What modifications are necessary to the airframe and or instrument panel to add an autopilot to a light airplane (like a Cessna 172 or 182)?
I participated in a project to do this to a light aircraft some time ago. The requirements will vary on the basis of the type of autopilot you select and the requirements of the STC that comes with it, and which allows it to be installed on your aircraft. There might (and likely will be) one or more of the following requirements: panel mods, electrical mods, servos need to be installed (type, placement and installation procedure as per the STC), there will also have to be ground and flight testing, and if you intend to interface the unit to navigation equipment, then it gets really interesting. It all depends what other avionics you already have in your aircraft. You may or may not have to upgrade your existing equipment to achieve that. 

To get a complete answer to your question for a specific aircraft you need to go to a reputable avionics shop that has done the work before (read: has references) and pose the question to them. 

You should also expect to have to pay for a detailed quote and time estimate -- it takes quite a bit of work to prepare that, and avionics shop owners have to pay bills and put food on the table, just like you. :)