A dealer offers 0.9% financing on a new truck. You have a home equity line of credit sitting at 8%. The line of credit is more expensive, so the dealer money wins. Except the comparison isn't finished, because a promotional rate almost always has a price attached.
Financing at 0.9% is money the manufacturer is buying down, and they'll usually make you choose. Take the promotional rate, or take the cash rebate. Not both. So the real question is what the rebate pays you in dollars against what the cheap rate saves you in dollars.
Run it. A $45,000 truck over 60 months. Take the 0.9% and you finance the whole $45,000 and pay about $46,040 by the end. Take a $3,000 rebate instead and finance $42,000 off your line of credit at 8%, and you pay about $51,110. The dealer's money wins by roughly $5,000.
Now make the rebate bigger. Same truck, a $6,000 rebate, same 8% line of credit. You'd finance $39,000 and pay about $47,460 total. The dealer rate still wins, by about $1,400. It takes a large rebate and a cheap line of credit to flip this, which is why the promotional rate is usually the right answer when it's offered on the vehicle you were already buying. I ran the same kind of comparison on camera, borrowed money against a bank loan on a car, and ended up changing my own answer partway through it. The arithmetic decides this, not the principle.
Three things change the answer. The promotional rate is reserved for top-tier credit, and the tier you actually qualify for gets decided at the finance desk rather than in the ad. The promotional term is often shorter, 36 or 48 months instead of 72, so the payment runs higher even though the rate is lower. And a used vehicle or a private-party sale has no manufacturer money behind it at all, which is exactly where the line of credit does its best work, because you show up with certified funds and negotiate like a cash buyer.
When you do use the line of credit, the loan comes with no payment book, so write the schedule yourself before the money moves. Pick the monthly amount and the payoff date, set the transfer to run automatically, and treat it like any other note. A vehicle bought off a line of credit with no payback plan turns into a permanent balance, and paying yourself back is the whole habit.
One thing I wouldn't do is take the dealer's cheap rate and then let the truck payment eat the surplus that was funding your chunks. A 0.9% loan is cheap money and it's still a payment every month. Add it to the monthly plan before you sign and look at what's left over for the mortgage, because your real monthly surplus is the number that decides how fast this whole plan moves.