To produce a back-dated stock list (i.e. 31/12/17) go into Accounts > Nominal Ledger.
On Reports select Scheduled Nominal.
Enter the stock nominal - if you have more than one branch and sell New and Used vehicles you can enter the nominal code as : *.3*.60.80.
I would recommend doing it branch by branch as there may be a lot of data which will cause the report to take longer to run. For example, 1.3*.60.80.
Enter the required date that you would like the Stock List for, and then select Report.
This will produce a list of all stock for the date required.