TL;DR: You can sleep drunk in your car but you shouldn't give the impression that you're attempting to drive.
The German criminal code says, in section 316:
Whoever drives a vehicle in traffic (sections 315 to 315e) although they are not in a condition to drive the vehicle safely due to having consumed alcoholic drinks or other intoxicating substances incurs a penalty of imprisonment for a term not exceeding one year or a fine, unless the offence is subject to a penalty under section 315a or 315c.
Of course, the question is what exactly "drives" means (and whether "drives" is an adequate translation; to me, it seems like "drive" actually means moving the car, while the original German word "führt" could be interpreted as "controls")
A German administrative court ("Verwaltungsgericht") said in this decision:
Ein Fahrzeug führt derjenige, der es unter bestimmungsgemäßer Anwendung seiner Antriebskräfte unter eigener Allein- oder Mitverantwortung in Bewegung setzt oder das Fahrzeug unter Handhabung seiner technischen Vorrichtungen während der Fahrbewegung durch den öffentlichen Verkehrsraum ganz oder wenigstens zum Teil lenkt, wobei der etwa vorhandenen Motorkraft als Ursache der Bewegung keine Bedeutung zukommt.
translation by deepl.com:
A vehicle is driven by anyone who sets it in motion under their own sole or joint responsibility using its driving power as intended, or who steers the vehicle in whole or at least in part using its technical equipment while driving through public traffic areas, whereby any engine power present is of no significance as the cause of the movement.
This should mean that a car that doesn't move is not driven.
In this case (in German), a man sat in the driver's seat, started the motor and turned on the light, then stopped the motor again as his wife said it might be better to walk. Some officers in a passing police car saw that, and the man ended up being convicted; this conviction was overturned by the highest German court. So, as long as the car isn't moving, you're not driving, and §316 doesn't apply.
Still, running the motor can get you in some trouble with the police, and winning your case in court after several weeks or months of having your license suspended probably isn't what you'd call a favorable outcome. And indeed, several non-juristical websites give general advice that you can sleep in your car, drunk, if you make sure that it's unreasonable to assume you're making an attempt to drive. For example here, section 44, the magazine says (again translated by deepl and slightly edited to reflect the original formatting):
Item 44: "I'm not allowed to spend the night drunk in the car." Actually, this is permitted in principle. However, it must not appear that the vehicle was being driven under the influence of alcohol or is to be put into operation. If this is the case, the person sleeping drunk could be charged with attempted driving and thus an attempt to commit a criminal offense. However, if you follow the following two instructions, you are on the safe side.
Firstly: Never sleep in the driver's seat. The best (and usually most comfortable) place to sleep is on the back seat, but the front passenger seat is also fine, as the car cannot be started from there without further ado. And secondly: do not insert the ignition key. Not even to switch on the heating or air conditioning. The vehicle will then be more ready for operation than it should be. Cars with keyless go systems are permanently ready to drive, but the ignition should remain switched off to avoid misunderstandings.