right this is what you need to do
make out a bill, include everything you did to vehicle dont miss anything, itemise it to the last dot on an i!
make a copy for yourself, give the other to the customer
make customer aware he has until xx/xx/xx to pay again in writing with a copy for yourself
after the date has come inform customer that storage will be charged
if still not paid, start making bills for storage, daily weekly monthly etc,,, make sure you send bill to customer as well as a copy for yourself

if still no luck, begin court proceedings for your money if customer fails to present himself you will win, then after a total of 3 months in your possesion you can technically sell his car, i say technically because my ex boss technically sold a customers car this way but for whatever reason LEGALLY customer took him to court and was awarded the costs of his car back!