Bit more than a couple of times- he did the first two or three rounds then dipped in and out of the series- particularly appearing at the GP supports etc. As I remember, it was a very low-budget effort with Gruen often acting as his own mechanic.
Just dug out some old Donington ETCC & TT programmes and had a look at Racingsportscars.com to get a handle on the major regular ETCC Corollas, so prepare for Corolla pic overload....
The Nielsen/Hoyer Castrol car was very much the dominant Corolla early on around 1984/5/6- Kurt Thiim and Chris Hodgetts also had outings in the Castrol car in 86.
http://www.racingsportscars.com/etcc...-09-29-071.jpg
The other main Corollas at that stage were Milos Bychl's ASP Motorsport car, shared with a variety of Czech and Italian drivers (and interestingly Andrew Miedecke at the 85 TT) ...
http://www.racingsportscars.com/etcc...-07-28-072.jpg
...the German Robert Schumacher car- usually shared with none other than Hermann Tilke(!) and the Belgian IMC car of Fermine/de Liedekerke.
The number of Corollas increased in 86 with the likes of the Czech Georg Albers/Antonin Charouz car, the Swiss Bemani entry, the Lingmann/Holzl Scuderia AVUS car, some ETCC appearances by Chris Hodgetts, and IMC.
Was IMC the Belgian Toyota importer or something like that? If you look at Racingsportscars.com there always seem to have been loads of 'IMC' entries at Spa, including some of the regular cars from other teams
By the time of the WTCC in '87 the main players were Bemani, the Belgians, CHMS, Alber/Charouz etc. Apart from that there were the local entries at the 'flyaway' rounds in Australia, NZ and Japan- Team Toyota Australia, Bob Holden, plus a load of Japanese entries at Fuji
By '88 there were three main 2-car Corolla outfits in the ETCC:
the Belgian Fina cars for Guy Katsers/J-C Burton and Pierre Fermine/Serge de Liedekerke (think the rally driver you mentioned was J-P van der Wauwer or something like that)
http://www.racingsportscars.com/etcc...-05-29-122.jpg
the AAGRK Prague/Marlboro cars of Albers/Charouz and Eddy van Esch/Noel van der Eeckhout
http://www.racingsportscars.com/etcc...-05-29-126.jpg
and a pair from Peter Seikel- usually Seikel/Dagmar Suster in one and a variety of Italians in the other
http://www.racingsportscars.com/etcc...-05-29-124.jpg
Apart from that there was the Spanish Camac car...
http://www.racingsportscars.com/etcc...-05-15-129.jpg
...and a variety of others that dipped in and out of the series, such as Bob Holden at Spa, Stig Gruen, the fwd TOM'S GB car (plus a TOM'S car from Japan at Spa), CHMS etc. I'm pretty sure Gruen's car ran in Fina colours at Spa and the TT as well as the two Belgian Fina entries.
One interesting point is that apart from the Danish Castrol car, the majority of the European entries seemed to favour the 2-door saloon version rather than the 3-door hatch/coupe favoured by the BTCC Corolla entries. Was the 16-valver ever sold in 2-door form in the UK?