Policy or terms with mobil installer?

Hello everybody and "Happy Thaksgiving Day"

Does someone know or have like you know  policy or rules when you start working in a customer car?for ex. when you work with alarms and you need to add relays or any additional parts to make the alarm or anything work ,or If something doen't work because the car has problem or the system of this car must to be bypass or something like that.

I don't know if you understand what I trying to said but I need any idea to make my own policy or however you can call, to cover you and is justify in case is  something not gonna work because you need extra parts or whatever to make it work!

If I need this is because sometimes customer said The alarm is not good or doesn't work the way that suppost to be and etc...

but is not the alarm problem is the car ! you know what I saying?

So while we're on the topic...any advice for someone thinking of "officially" starting a side-buisness out of this?    I've been doing CA&E for +10years, and security 5 or so.  I recently got back into doing remote starters for "friends" but now its starting to get a little more then that (a very good thing, as this is something I like to do - and it pays :) ).   

I too have been looking for some type of policy & procedure sort of thing, and have been getting concerned about the liabilities involved in playing with someones primary mode of transportation as my involvement in this increases.  is it possible to be insured if I am not officially licensed /certified/Inc'd. etc.  Im looking into getting a tax ID# presently and taking that route - just not sure what ill encounter in the process.  Any tips would be much appreciated :) 

Thanks! / OC