I don't like the stock position. Worse yet, I don't like the RC car look. Opels don't get much respect due to size. Looking like a toy... well, you know what I mean.
The windshield ants. worked good but cost too much. They used to sell kits until GM ended production.
Some Japanese cars use(d) ants. in the trunk lid. My '84 Mitsibishi had one. I liked it but it hurt reception when you're far away from anywhere. Esp. if anywhere is toward the back of the car. Reception forward is better.
I bet you can find trunk lid ants. in junkyards. They hung on plastic clips, nothing fancy. There's plenty of room to hide one on a GT. Under the headliner would be best, if you're removing it.
Crutchfield used to sell powered ants. for the trunk. I don't know if they still do. I'm not talking retractable. I'm referring to boosted signal.
$50 buys a generic retractable ant. They don't last long in daily use, like 5-8 years. I've bought several, gave up, and use a solid ant. on the Nissan now ($5). No more scratching as the ant. goes up/down. No more busting ice off. No more $50.