Need some clarification. Do both you and your wife have iPhones?
As it stands--no phone will pair with your 2005 4.8.
Your phone pairs with the M5. What year is your M5?
Does either phone pair with the the 2006 4.8 or the 3.0? What year is the 3.0?

Pairing instructions have changed since Bluetooth was first offered. Still not the easiest thing to do but easier than the introduction. As an example, later models don't ask for the car bluetooth pass key they ask for the phone pass key, that is the source of the 0000 and 1234 iPhone default suggestions.

If your phone pairs with the M5 suggest you pair your wife's phone and write that process down and keep it in the glove box. Let's get something accomplished. Take your time and be sure the manual process works as written or what you wrote down is exactly what you did. I usually don't resolve problems like this on the first try so when I have found the solution I may not be exactly sure of what I did. I have gotten to the point that I write down every attempt so I can be sure I have a list I can use for process of elimination.

Compare the telephone instructions in the 2006 4.8 from those you wrote down for the M5. They may have changed but it is likely they will be close enough to the M5 that you can pair both phones

Then lets go to the source of your frustration the 2005 4.8. Compare what you wrote down for the M5 and 2006 4.8 to the instructions that are in your manual and to those earlier in the thread by Qsilver7 and hopefully pairing instructions for the 2005 will make sense from your success with the other vehicles. If not, please post what you tried on the 2005 that didn't work.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas