To travel to Brunei from Sabah you have to cross the border either sungai tujuh, sarawak or icqs megkalap (choice is yours)
Make sure you have a valid international driving license & the car rental allows you to cross the border because sometimes the car rentals simply won’t allow or they don’t have permission.
Checkout these two things before planning your travel by road, apart from your multiple entry visa.
Also, just to add in my personal experience, highways are not so great either in Sabah.
Wish you a wonderdul journey & feel free to ask any further info
Since, I’m based in West Malaysia, I’m not too familiar with the car rental places in Sabah, but I suggest you to make an arrangement before your arrival. Maybe you can talk to your travel agent.
For assisting you further, here I would like to tag some Local Guides who might be able to help you further @Cweng@StephenAbraham
If you happen to not yet venture on this trip and if you still insists on going by land. There are some buses that do cross border crossing. it’s a pleasant experience too. As i’ve done it once, the scenery along the routes is just amazing.
I just wanted to let you know that I have removed the Chinese language label of your post as it’s written in English. These types of labels are used so that posts in different languages can be found more easily by Local Guides who are speakers of these languages. You can learn more about language tags in the article: How do I find posts in my preferred language?.