Audi and BMW believe the DTM needs to outline its next set of regulations as soon as possible in order to secure the series' long-term future with a third manufacturer.
Mercedes will exit the DTM at the end of 2018, ending a 30-year association with the German touring car series.
The DTM is currently trying to attract a replacement, but ITR and series boss Gerhard Berger admitted to Autosport that the search for a third manufacturer has been difficult.
Autosport understands that Audi has been willing to help the series find a Mercedes replacement by demonstrating the DTM's affordability.
Berger said the next eight weeks would be crucial in finding a third manufacturer for 2019.
Audi DTM boss Dieter Gass said Audi would be willing to commit to a bridging year in 2019 as long as the series' future after then was clearly defined.
"I'm sure it is a concern [having two manufacturers]," Gass told Autosport.
"It has been said several times that DTM with two manufacturers is difficult to imagine.
"In the current situation we could potentially bridge a year but in the long-term, we need a third manufacturer.
"I think at one stage you need to make a plan for someone who wants to come in and it needs some advance planning and it's the same for everyone. Around mid-year, you need an indication."
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BMW's director of motorsport Jens Marquardt stressed that new manufacturers would not join without clearly defined regulations.
"I think at the moment the focus needs to be to get everything properly established so that potential newcomers really know what they get themselves into," said Marquardt.
"Number one in that direction is the regulations, to really finalise them.
"For sure, somewhere during the year, decisions need to be made. But the focus needs to be getting people on-board and not looking elsewhere."
Berger said he understood BMW and Audi's concerns.
"You need it [the regulations] as you have a lead time on the technical side and you need to know what you are doing," he said.