Welcome to the forum, jenifben,
In a layout from the Customer Details Table just add a field from the 'Job Details' table and you will see that Filemaker is happy to 'see' all the connections all the way along the chain:
Customer Details -> Vehicle Details -> Job Details.
Use a portal if you want to see several records from the Job Details table.
You can directly relate the Customer Details Table to the Job Details (if you don't want to go through the Vehicle Details table). Do as you are doing, dragging the CustomerID fields to match in both tables. Filemaker will create a second 'Occurance' of the Job Details Table, because when you link to it from the Customer Details Table, Filemaker has to know whether you mean to use the first relationship to find related records, or the second. You tell it which you mean by choosing the 'Table Occurance Name' from the top of the field listing in setting a 'Calculated Field', for example.
However, if you are only going to use this new relationship to 'tunnel' the CustomerID all the way through, there may not be any advantage (unless, for example, some Jobs do not involve any Vehicle.)
See if this starts you going down the right lines, then post again if you want more details.
Did you get your database working the way you wish?
Thanks Sorbsbuster, that got me going along the right track and I think I almost have this database thing sorted.