Ever since Techniques d'Avant Garde (TAG), manufacturers of high-tech items such as ceramic turbochargers for Formula One cars, acquired Heuer in 1985, it has relied on ETA for both its Quartz and mechanical movements, even though until about 2000 it was mainly a Quartz relationship.
In early 2000s (after LVMH's acquisition in 1999), TAG Heuer gave a very strong push to being back mechanics. From 2001 to the 2006/7 period, TAG Heuer had grown the mechanical part of the brand at a very fast rate and from a few thousand movements back in 2001 to around 200,000 mechanical movements by 2006/7, mostly from ETA.
By now, TAG Heuer has been transform from a mainly Quartz brand to a brand with both and having almost only mechanical movements for their Chronographs.
Today Swatch Group is still a very important supplier to TAG Heuer for mechanical movements. But it is no longer the only one nor the largest supplier. It is now as big as the second-largest supplier, Sellita (as Sellita started their SW-500 programme quite recently, they supply non-Chronographs movements to TAG Heuer mainly).
p.s. Worth to know (it is said) the so-called 100% in-house mechanical chronograph movement of TAG Calibre 1887 is a actually a re-work of Seiko 6s37 calibre under license => Responses from Babin Jean-Christophe, CEO of TAG Heuer.
data source: calibre11.com