Winter testing has been completed for the VW ID.EVERY1, the first vehicle under a joint venture between Rivian and Volkswagen Group to be equipped with the EV maker’s software and electrical architecture. That’s not just progress toward getting this vehicle into customers’ hands; it also unlocks another $1 billion investment from Volkswagen Group into Rivian.

About $750 million is coming in the form of an equity investment. The other $250 million is either equity or convertible debt, depending on which prototypes Volkswagen Group provided to Rivian for testing. (The companies did not make this immediately clear.)

The German automotive giant has already invested a little more than $3 billion in Rivian as part of the joint venture. And there’s more to come. Rivian will be able to borrow up to $1 billion from Volkswagen Group starting in October. Rivian also gets another $460 million equity investment from Volkswagen after the first vehicle goes on sale using the joint venture’s tech. All told, the deal could be worth as much as $5.8 billion to Rivian.

The winter testing milestone payment has been delivered just months before Rivian starts selling the R2 SUV, which founder and CEO RJ Scaringe has said is “maybe the most important thing we’ve launched to date.” Rivian is banking on a very fast scaling of R2 production and sales.