If you have made a mistake when invoicing a vehicle, there is a utility called Stock Unit Sales Reversal Process. In other words, you can reverse/cancel the invoice by raising a Credit Note against it.
If the menu does not exist, you may have to add it via Menu Maintenance.
Specify the invoice number that you want to reverse.
Tick Purchase Flag Confirmation Box.
Press REVERSE SALES.
Follow the instructions and you should be able to print a Credit Note. All the GL Postings related to this invoice will be reversed.
NOTE
This should work quite smoothly if you haven't processed anything against the stock unit after you invoice. If you go to Stock Unit Maintenance for the stock, look at the Cost Transaction Tab. There shouldn't be any Ticked Boxes unser 'After Sale Indicator' Column. This column indicates whether there are any transactions done AFTER you Invoice the stock. In this case After Sale = After Invoice (not to be confused with Aftersales).
If you have any After Sale transactions against this stock, you will have to reverse those first before you are allowed to cancel the invoice.
For example, if you have processed a PO against the stock after invoicing, you need to cancel the PO before you are able to reverse. Another example (which is more common) is if you have issued a payment/cheque against the stock after it has been invoiced, you'll need to cancel the cheque first.
If all these doesn't work, there is actually a menu parameter which allows you to reverse sales with an After Sale transaction.
The parameter is {AllowWithAfterSaleTrans=Y}
If you take this approach, you may need to manually post journals to completely reverse any outstanding balances.
Contact me if you have difficulty to reverse any invoices.