Customer - name, address, phone, and email.
Cars - VIN, model, year, color, engine, and notes. One to many link with Customer.
Jobs - Work done, date received, date shipped, price charged, paid status, and notes. One to one link to Customer, one to many link to Cars.
Correspondance - Date, name, communication method, reason, notes, and employee. One to many link to customer. One to one link with Jobs.
Employees - Name, address, SSN, salary, speciaties, phone, and title. One to many link with Correspondance.
For lookup tables, I would create a lookup table for all the possible jobs that could be done (engine swaps, body kits, repairs, etc). And maybe create another for the type of correspondance.
For validation, validate the phone numbers, SSN, and emails.
Instead of Employee, I was going to also do a table for Inventory. Engine swap kits, suspension and brake componants, consumables (oil, grease, hardware). The Job table could use the Inventory table to fill in Jobs. Instead of Correspondance, you could do a Job_Parts table. This would list all the parts (from Inventory) that were used for a Job.