Create a loan from the car business to the RE business. Then have the RE business deduct the interest payments following the regulations. The car business will record the income but also has interest expenses as the funds came from an LOC.
You really want to keep the two business separate and when ever they do a deals between themselves document it correctly. Pretend that two strangers were doing the deal and document the same as you would in that situation.
REI is correct here. You would have a deduction and payment of the LOC and interest related to it paid by the real estate company and paid to the car dealer. The car dealer will show income for the interest received and an offset for interest expense as it makes the payment to the LOC.